1. Field of the Invention
The present invention relates to a display apparatus having a picture-in-picture or picture-out-picture function for superimposing a sub picture on a main picture being displayed on a screen of a television receiver or the like.
2. Description of the Conventional Technique
FIGS. 12(A)-12(D) show some examples where a sub picture is displayed in a predetermined area of a main picture. In FIG. 12(A), a main picture is displayed in a left portion of a screen, and one sub picture is displayed at its lower right end. Next in FIG. 12(B), there are displayed three sub pictures of individually different channels. In this example, the images of three channels are visually represented as indexes. The display method employed in the above two examples is generally termed PoutP (picture-out-picture) since any sub picture is positioned at the end portion of the main picture.
Meanwhile in FIGS. 12(C) and (D) is employed another display method termed PinP (picture-in-picture). In the example (C), a main picture is displayed in a zoom mode with its upper and lower end portions cut and a sub picture is displayed with partial insertion in the lower right area of the main picture. Further in the example (D), sub pictures are displayed at the four corner areas of the main picture respectively.
FIG. 13 is a circuit block diagram of a conventional display apparatus for superimposing a sub picture on a parent-picture as mentioned. In this example, a mixer 101 mixes a sub picture signal with a main picture signal and supplies the mixed signal to an aspect ratio converter 102. Then the aspect ratio converter 102 converts the aspect ratio of the input signal and supplies its output to a double speed converter 103, which converts the input video signal into a double speed signal and outputs the same to a mixer 105 via a switch 104. Subsequently the mixer 105 delivers the input video signal from the double speed converter 103 directly as an output, or mixes a sub picture signal with the input main picture signal of HD-MAC format received via the switch 104 and then outputs the mixed signal therefrom.
Now the operation of the above conventional apparatus will be described below with reference to a timing chart of FIGS. 14(a)-14(e). The mixer 101 is supplied with the parent-picture and child-picture video signals of the
or NTSC system. The substantial picture signal included in such composite video signal is so set as to have, e.g., 720 pixels per horizontal scanning period H. The mixer 101 curtails the pixels of the sub picture to, e.g., 240. And the data of the 240-pixel sub picture (FIG. 14(b)) is inserted in a predetermined region of the 720-pixel main picture (FIG. 14(a)) to thereby produce a new video signal (FIG. 14(c)).
The data thus obtained is converted by the aspect ratio converter 102 into a signal of a desired aspect ratio. More specifically, in case the aspect ratio of a display unit (not shown) connected to the rear stage of the mixer 105 is 16:9, the video signal of an aspect ratio 4:3 supplied from the mixer 101 is converted into a video signal of an aspect ratio 16:9 (FIG. 14(d)). The converted video signal is inputted to the double speed converter 103, which then produces a video signal of a double field frequency converted as shown in FIG. 14(e). The video signal thus produced is supplied via the switch 104 to the mixer 105, from which the signal is directly outputted to and represented visually on the display unit.
FIG. 15 shows the principle of such double speed conversion. The PAL or NTSC video signal is in a 2:1 interlaced format of 625 lines at 50 Hz, and its aspect ratio is set to 4:3. The one-frame video signal of 625 H consists of two video signals of an odd field and an even field each composed of 312.5 H. The double speed converter 103 produces, out of the odd-field video signal, two double-frequency odd-field video signals of 313 H and 312.5 H. The converter 103 further produces two even-field video signals of 312 H and 312.5 H by doubling the frequency of the even-field signal of 312.5 H.
Consequently, as shown in FIGS. 16(a)-16(d), the odd-field video signal of 312.5 H is displayed in succession to the odd-field video signal of 313 H. And subsequently the even-field video signal of 312 H is displayed, which is followed by the even-field video signal of 312.5 H. Since the field frequency is doubled as mentioned, it becomes possible to prevent occurrence of flicker.
Meanwhile the HD-MAC video signal based on the European HD TV standard is in a 2:1 interlaced format of 1250 lines at 50 Hz, and its aspect ratio is set to 16:9. In case the HD-MAC video signal is used for a main picture, it is impossible to employ the mixer 101 in common to the PAL or NTSC system and the HD-MAC system since the number of lines in the former and that in the latter are fundamentally different from each other. Therefore, in using the HD-MAC video signal for a main picture, the circuit configuration is so contrived that the parent-picture video signal is supplied to the mixer 105 via the switch 104, and a child-picture signal is mixed with the parent-picture signal in the mixer 105.
In displaying a sub picture to a main picture in the PoutP mode, a child-picture video signal of 240 pixels is added to a parent-picture video signal of 720 pixels, as shown in FIG. 17(a). It follows therefrom that the length of the mixed signal is rendered greater than the length of 1 H. Accordingly, for double speed conversion of such signal, there exists the necessity of providing two field memories in the double speed converter and writing the data therein alternately per line. For this purpose, memories for the data of two fields are required to consequently raise a problem with regard to an increase of the production cost. In an attempt to eliminate such disadvantage, the video signal shown in FIG. 17(a) is so processed that its aspect ratio is converted as shown in FIG. 17(b) to remove the overlap on the time base, and then double speed conversion is executed. This process enables a single field memory to be sufficient for the double speed conversion.
However, since the number of pixels of the 1 H data inputted to the double speed converter 103 is 960 (=720 +240), the storage capacity of the memory employed in the double speed converter 103 needs to be 4/3 (=960/720) times in comparison with 720 pixels required for the display in the picture-in-picture mode. Consequently the problem of the high production cost is still left unsolved.
Furthermore, in one case of using the HD-MAC video signal for a main picture and another case of using the NTSC or PAL video signal for a main picture, the numbers of lines in such two cases are widely different from each other to eventually fail in achieving common use of the circuit which mixes a sub picture with a main picture, whereby circuits of two systems are needed and increase the production cost.
In addition, if the frequency-converted video signals are employed for discrimination between the odd and even fields of the video signals after conversion of the field frequency, it becomes necessary to discriminate among 312.5 H, 313 H, 312.5 H and 312 H, to consequently requiring a greater circuit scale, hence causing difficulties in realization of stable discrimination.